1. Field of the Invention
The present invention relates to a hot melt composition and more particularly to a hot melt composition, which inhibits brittleness at low temperature and fluidity at high temperature and is superior in flexibility, and which has adhesion and easy release to the adherend and also has excellent suitability for buffer material, vibration-isolating material or industrial sealing material.
2. Description of the Prior Art
With recent rapid progress of a polymer synthesis technology, functional plastics and elastomers, which are superior in balance between cost and performance, have been intensively marketed in place of a conventional material in any industrial field.
For example, in the metal substitution filed to which heavy-duty characteristics at high temperature are required, a share of thermoplastics having excellent moldability has rapidly been enlarged, excluding a part of thermosetting resins. On the other hand, in the field of a rubber product, a conventional vulcanized rubber produced by kneading a raw rubber, a reinforcer and a vulcanizing agent and then passing through a step of molding and vulcanization, using much labor and energy is obliged to be withdrawn or isolated from a part of application fields at present because of the advent of a thermoplastic elastomer which can be molded or shaped by using a molding machine for thermoplastics.
Heretofore, various sealing materials have been used to maintain sealing properties of the bonded portion of containers, building structures, various molding machines or the like.
As the sealing material, materials having characteristics such as rubber elasticity are often used, and formerly, molded articles of vulcanized rubber, soft vinyl chloride resin, polyurethane or the like was used. Recently, hot melt type elastomer polymers have been used.
Furthermore, in the application field for sealing material to which rubber-like properties are required at high temperature, cost reduction is not effected, since it is troublesome to fit vulcanized molded articles and synthetic resin foamed articles and it is also difficult to perform automation. Therefore, attempts for practical application of a hot melt type sealing material of a thermoplastic elastomer has gradually been made which can be molten and fit at a job site.
In a conventional hot melt mixing system, since a low-molecular weight or moderate-molecular weigh elastomer is basically used as a base polymer, the shape becomes unstable at high temperature range and fluidization can not be avoided. It is a technical common knowledge to add a compound of wax having a structural viscosity in order to prevent fluidization. However, when wax is added, the flexibility is considerably lowered, resulting in poor balance between the heat resistance and flexibility. Therefore, such an elastomer is not suitable for applications such as sealing material. Since the moderate/low-molecular weight elastomer is used as a base, initial tack is excellent. However, the release properties become poor to cause a problem that it becomes difficult to release from the adherend after use.
In addition, since the molecular weight of the base polymer is low, the softening temperature is low and the heat resistance is poor. When the liquid softener is added to impart the flexibility, mechanical characteristics at high temperature are lowered to cause a phenomenon that fluidization is caused by its own weight in the range at 50.degree. C. or higher or large compressive permanent strain is remained when compressed. These characteristics are not suitable for applications such as sealing material to which sealing properties at higher temperature are required.
In an elastomer hot melt mixing system, process oil is generally added to impart the flexibility. As the amount of the process oil increases, the compressive permanent strain becomes larger. Accordingly, also in this case, a balance between the flexibility and compressive permanent strain is poor and the suitability as the sealing material is lowered.
Even in case of a styrene block copolymer which is considered to have excellent restoring properties among hot melt type systems, sufficient sealing properties can not be maintained, since the polymer having the composition used at present exhibits considerably large compressive permanent strain at high temperature. Therefore, it can not be applied to practical use.
In a hot melt composition using a conventional styrene thermoplastic elastomer, there can be used, as a basic mix, those prepared by using an elastomer having an intrinsic viscosity [.eta.] measured in decalin at 135.degree. C. of less than 1 dl/g as a base and adding a liquid softener such as oil, liquid rubber or the like in the amount of about 20 to 200 parts by weight based on the 100 parts by weight of the elastomer.
As other sealing materials using the elastomer, for example, Japanese Patent Kokai Publication No. 4-110381 discloses a hot melt gasket composition comprising 50 to 100 parts by weight of a reactive elastomer of a carboxyl-modified styrene-ethylene-butylene-styrene block copolymer or a styrene-butadiene-styrene block copolymer, and 50 to 250 parts by weight of a softener comprising a process oil.
This invention discloses as follows. That is, when the amount of the softener is less than 50 parts by weight, the viscosity of the mixture becomes extremely high, resulting in poor machinability and workability. On the other hand, when the amount exceeds 250 parts by weight, the workability and heat resistance become poor.
In the Examples, the followings are specifically described. That is, a styrene block copolymer used as a reactive elastomer is a carboxyl-modified styrene thermoplastic elastomer having an [.eta.] value of 0.67. A composition obtained by mixing a two-fold amount of paraffin oil with this styrene block copolymer and further mixing a synthetic wax, a low-density polyethylene, an ethylene-vinyl acetate copolymer, a nonionic surfactant or silicone oil for releasing as an optional component is superior in close-contact and adhesion to an iron plate.
As described above, an object of the above prior art is to provide a gasket composition for enhancing close adhesion to an iron plate, and the used reactive ethylene monomer is characterized by being a comparatively low-molecular weight styrene block copolymer.
As described above, since the conventional sealing material and caulking material including those of the above prior art use a moderate- or low-molecular weight elastomer as the base, basically, not only the softening point is low and fluidization arises at high temperature, but also the compressive permanent strain, particularly compressive permanent strain at high temperature, is large. Therefore, they are not preferred as the sealing material in view of the suitability.
To prevent fluidization, wax for imparting a structural viscosity has been added. In that case, the flexibility is lowered, resulting in a mix having properties which are brittle and liable to be broken.
Large compressive permanent strain means poor restoring force against loading. Since such a material is plastically deformed in the loaded state, when vibration is applied with a lapse of time, there arises a problem that a gap is gradually formed between the material and a member to be sealed and a good sealed state can not be maintained for a long period.
With recent requirement of recycling of materials and gentleness to global environment, objects for disposal as an industrial waste must be discarded after separating them by the kind of materials as possible. However, it is difficult to separate a gasket adhered firmly onto the adherend such as iron member, like the above prior art. Therefore, the iron member and gasket, which are respectively made of different materials, are discarded as they are without being separated, resulting in one of causes of environmental disruption.